This invention relates generally to a novel rosin modified styrene/acrylic binder resin and more particularly to such a resin which is useful as a component of a reprographic dry toner composition.
In electrophotographic printing, generally a uniform electrostatic charge is applied to a photoconductive insulating layer and the resulting charged surface is selectively exposed to electromagnetic radiation so as to dissipate the charge in those areas exposed to the radiation, thereby producing an electrostatic latent image. The resulting latent image is subsequently developed by depositing (typically by use of the well known "cascade" method) a finely divided electroscopic developer material, referred to as toner, on the electrostatically formed image. Generally speaking, the charged toner particles will have a charge opposite to the residual electrostatic charge image, so that the toner particles adhere to the charged areas to form a visible image. This image may be fixed in situ on the support or transferred to a secondary support surface, and the transferred image permanently affixed to the secondary support surface.
Two component developer mixtures are conventionally used to develop electrostatic latent images, comprising a pigmented resinous toner powder and a carrier component wherein the carrier component is substantially larger in size than its toner complement. The toner particles, which are generally made of a fine pigmented resinous material, are charged triboelectrically by rubbing against the carrier particles causing them to adhere electrostatically. The composition of the developer mix is chosen such that the toner particles will acquire an electrostatic charge of a polarity opposite to that of the electrostatic image to be developed. As a result, when the developer is brought into contact with the electrostatic latent image, the toner particles are attracted from the carrier particles and selectively deposited onto the electrostatic image by the electrostatic charge of the image. The powder or toner image that is obtained is either fixed in situ on the surface of the image bearing substrate, or the powder image is selectively transferred to a receiving surface to which it is then fixed. The fixing process can reflect any one of several approaches such as pressure fixing, vapor fixing or heat fusing, depending upon the specifics of the particular system.
In another form of development utilizing a developer mix comprising a carrier and toner component, a developer composition containing toner and magnetic carrier particles is transported by a magnet. The resulting magnetic field causes alignment of the magnetic carrier into a brush like configuration. This magnetic brush is engaged with the electrostatic image bearing surface, and the toner particles supported on the brush like configuration are drawn from the "brush" to the latent image by electrostatic attraction. Thus, a developer mixture may be provided comprising a toner material and a carrier material which consists of particles which are magnetically attractable. Such a configuration is generally referred to as a magnetic brush development system (as opposed, for example, to the cascade method which often uses glass beads as the carrier).
When the non-exposed portion of the surface of the photosensitive layer, prior to the developing step, is charged with a negative electric charge after charging and exposure, a positive picture cannot be obtained unless it is developed with a toner which is charged with a positive electric charge. However, most resin powders imparted with friction by the cascade method (using glass beads as the carrier), or by the magnetic brush method (using iron powder as the carrier) are charged with a negative charge, and very few of them are charged with a positive electric charge.
In order to impart to the toner either a strong negative or positive charge, it is necessary to add a charge control agent. Typically, such charge control agents comprise any one of a number of known dyes. Because the vast majority of known toner compositions have inherently negative charges, large amounts of charge control dyes (e.g. basic dyes) must be used to provide a strong positive charge (e.g. about +10 to +12) to the toner when a positively charged toner is needed (as is often the case). It will be appreciated that the addition of such positive charge control agents will add additional costs in materials acquisition and processing. Accordingly, there is a perceived need for a toner composition which has an inherent positive charge and so requires relatively smaller quantities of a charge control agent for developing a positive picture.
Another well known problem with conventional toner compositions is that the charge control additive both does not admix well in the toner compositions, nor does it disperse uniformly therein. As a consequence, the surfaces of the resulting toner particles are not charged uniformly, either negative or positive, thereby causing fogging or trailing phenomena at the development stage; and thereby preventing the obtaining of clear and sharp developed images.